1. Field of the Invention
The present invention relates to an electron probe X-ray analyzer for performing elemental analysis using characteristic X-rays, such as an electron probe microanalyzer (EPMA) or analytical scanning electron microscope (analytical SEM) equipped with a wavelength-dispersive X-ray spectrometer (WDS) and an energy-dispersive X-ray spectrometer (EDS). More particularly, the invention relates to an analytical technique for identifying the composition of a sample.
2. Description of Related Art
EPMAs and analytical SEMs are instruments for analyzing samples by directing a sharply focused, accelerated electron beam at the samples and using the produced characteristic X-rays. These instruments are all known as electron probe X-ray analyzers. X-ray spectrometers attached to electron probe X-ray analyzers for spectrally analyzing and detecting X-rays produced from samples are classified into two major categories: WDS (wavelength-dispersive spectrometer) and EDS (energy-dispersive spectrometer). The WDS is a spectrometer for detecting with an X-ray detector only X-rays satisfying the Bragg reflection conditions by the use of an analyzing crystal. An X-ray spectrum is obtained by measuring the intensity of X-rays incident on the analyzing crystal, the intensity varying according to the incident angle of the X-rays. In contrast, the EDS is a spectrometer for obtaining an X-ray spectrum by converting X-rays incident on a semiconductor detector, such as a PIN detector into an electrical signal, guiding pulses having heights proportional to the energies of the incident X-rays to a multichannel analyzer, and accumulating the number of pulses for each of channels corresponding to X-ray energies.
Because of the difference in principle and structure between both kinds of spectrometers, analyses performed using WDS and EDS, respectively, have their features. For example, WDS can perform analysis with high wavelength resolution, high P/B (peak/background) ratio, and high count rate capability. Therefore, WDS is adapted for low-concentration analysis and analysis of chemical-bonding states. However, in order to analyze plural elements at the same time, it is necessary to equip corresponding plural spectrometers. Furthermore, plural kinds of analyzing crystals need to be mounted on each one spectrometer. Therefore, WDS has limited capabilities of performing easy and quick analyses.
Meanwhile, EDS can analyze multiple elements at the same time with a single detector. Furthermore, EDS has the advantage over WDS that the detector is not required to be driven mechanically for X-ray spectroscopy. However, EDS is inferior in energy resolution (corresponding to wavelength resolution of WDS) and P/B ratio of X-ray spectra to WDS. Additionally, in principle, EDS has the restriction that the efficiency at which only a certain X-ray is detected cannot be enhanced because of simultaneous analysis of multiple elements with a single detector. Therefore, EDS has restricted capabilities of performing low-concentration analysis and analysis of chemical-bonding states.
Therefore, taking account of the features of WDS and EDS, WDS is mainly fitted to EPMA while EDS is chiefly fitted to analytical SEM. In addition, many instruments are equipped with both WDS and EDS to make effective use of both WDS and EDS and to improve the analyzing capabilities and operability of the whole instrument. One known system has both a WDS system and an EDS system in a simple manner, and each system is separately operated to perform analysis. In another known system, WDS and EDS are controlled in an interrelated manner by a single control system to perform analysis (see, for example, Japanese Patent Laid-Open No. H2-47542). This system may be termed a “WDS/EDS combined system”.
A technique for identifying a compound present at a point of analysis from the results of quantitative analysis of a sample using EPMA or the like is disclosed in Japanese Patent Laid-Open No. 2000-266700. A technique associated with quantitative analysis of a particulate sample using EPMA or the like is disclosed in Japanese Patent Laid-Open No. 2001-27621. In this technique, a chemical type is judged from the features of the chemical composition at a point of analysis. The chemical type is made to correspond to the position and morphology of the point of analysis and is displayed.
Where quantitative analysis is performed using EDS alone, if the sample contains many elements in trace amounts, it is difficult to obtain accurate quantitative analysis results because of limitations in energy resolution and P/B ratio of EDS. Therefore, where quite highly accurate analysis results are required even from comparatively low concentrations of elements such as found in geological and mineralogical applications, if an instrument having both WDS and EDS is used, analysis will be eventually performed using only WDS.
When a relatively low concentration of elements is analyzed quantitatively by WDS, attention must be paid especially to the position of the wavelength at which the background intensity is measured (hereinafter may be referred to as the background intensity measurement wavelength). In particular, if the characteristic X-rays of other coexisting elements (known as interfering lines) are present near the background intensity measurement wavelength, it is impossible to find the intensity of the characteristic X-rays accurately by the effects of the interfering lines. This deteriorates the reliability of the results of the analysis. Where plural points of analysis are analyzed quantitatively using WDS alone, if the main components at plural points are similar, it is not difficult to determine the background intensity measurement wavelength without being affected by the interfering lines, for example, by the use of the technique disclosed in Japanese Patent No. 3,547,310. Where the interfering lines are X-ray diffractions of the second or higher orders (higher-order reflections), the higher-order diffraction lines (interfering lines) can be removed by a pulse height analyzer (PHA) equipped to the X-ray counter circuit (see, for example, Japanese Patent Laid-Open No. H2-25787).
Meanwhile, with respect to modern electron probe X-ray analyzers, such as EPMA, automation of analyzing functions using computer control is in progress. Therefore, many analysis points to be analyzed, for example, quantitatively, are stored in memory in advance. Measurement and arithmetic processing, such as calculations for quantitative corrections, can be performed automatically. However, such many analysis points often contain plural phases having different main constituents. In such cases, interfering lines appear differently. In the past, when such many analysis points are analyzed continuously, it has been possible to have only one set of analytical conditions (hereinafter may be referred to as the PHA operating conditions) under which the pulse height analyzer (PHA) is operated to remove the background intensity measurement wavelengths of the elements and higher-order diffraction lines creating interfering lines. Therefore, the human operator has searched for analytical conditions not affected by interfering lines at any analysis point, e.g., by preliminarily measuring some probable analysis points of plural phases, to determine one set of PHA operating conditions. This work placed a heavy burden on the operator if the technique disclosed in Japanese Patent No. 3,547,310 or Japanese Patent Laid-Open No. H2-25787 is used. In addition, if there is an analysis point having unexpected composition during analysis, there is the possibility that correct analysis cannot be performed.